Spruce-fir moss spider (Microhexura montivaga )

Spruce-fir moss spider. Credit: USFWS

Federal Status: Endangered, February 6, 1995

Description: The spruce-fir moss spider is one of the smallest members of the primitive suborder of spiders popularly referred to as "tarantulas." Adults of this species measure only 0.10 to 0.15 inch (2.5 – 3.8 millimeters) (about the size of a BB). Coloration of the spruce-fir moss spider ranges from light brown to yellow-brown to a darker reddish brown, and there are no markings on its abdomen.

Habitat: The spruce-fir moss spider only lives on the highest mountain peaks in the Southern Appalachian Mountains of western North Carolina and eastern Tennessee. The high elevation forests where this spider is found are dominated by Fraser fir with scattered red spruce. This forest type is commonly referred to as spruce-fir forests. The typical habitat of this spider is damp, but well-drained moss mats growing on rocks and boulders in well-shaded areas within these forests. The moss mats cannot be too dry (the species is very sensitive to desiccation) or too wet (large drops of water can also pose a threat to the spider).

Distribution: The spruce-fir moss spider is limited to a handful of mountains in western North Carolina and eastern Tennessee.

Map of Spruce-fir moss spider distribution in North Carolina.

Threats: The surviving populations of the spruce-fir moss spider are restricted to small areas of suitable moss mats on a few scattered rock outcrops and boulders beneath fir trees in the spruce-fir forests. Destruction of the moss mats (or even a portion of the mats) or damage to the surrounding vegetation shading the mats could result in the loss of the entire population or even extinction of this species.

In recent years, Fraser fir trees throughout the Southern Appalachian Mountains have suffered extensive mortality due to infestation by the balsam wooly adelgid, a nonnative insect pest believed to have been introduced into the United States from Europe. Most mature Fraser firs are easily killed by the adelgid, with death occurring within 2 to 7 years of the initial infestation. The remaining trees become more susceptible to exposure, wind, and storm damage. The red spruce trees are not damaged by the insect.

Furthermore, during the past century, most of the Southern Appalachian spruce-fir forest has suffered extensive changes and declines in size and/or vigor because of past logging and burning practices, storm damage, air pollution, climate changes, disease, insect damage, and exposure shock.

As the spruce-fir forest decreases in health and size, the death and thinning of the tree canopy results in locally drastic changes in the microclimate, including increased temperatures and decreased moisture. This leads to the drying of the moss mats on which the spider depends for its survival. The spruce-fir moss spider is very sensitive to desiccation and requires climates of high and constant humidity. As the mats dry out, so does the spider.

Critical Habitat: Critical Habitat for the spruce-fir moss spider was designated on July 6, 2001. For specific information on areas designated as Critical Habitat and the primary constituent elements defined for this species, please refer to the Federal Register notice (66 FR 35547 - 35566).